It is often necessary to be able to detect a signal in an optical waveguide by extracting a portion of the light being guided. For example, it is useful to be able to distinguish between live (signal carrying) and dark (no signal) optical waveguides, to be able to extract a signal for sampling, to perform an approximate measurement of the guided optical power, to perform accurate differential measurements of the guided optical power before and after a possible localized loss such as a splice, and to determine the direction of traffic. It is desirable to perform these operations without disrupting, overly attenuating or distorting the signal, especially in telecommunications systems, as the margins with which many such systems operate often do not permit more than about one decibel of additional attenuation without the risk of inducing errors to the transmission, thereby leading to potential traffic interruptions and even system shutdowns.
It is well known that light can be extracted from an optical waveguide such as an optical fiber by bending it so that a fraction of the guided light leaks from the waveguide. The extracted light is known as the bending loss. Devices for bending optical fibers to extract light in this way are often referred to as “optical fiber taps” and may use “micro-bends” or “macro-bends”. U.S. Pat. No. 5,708,499, for example, discloses the use of both micro-and macro-bends,
However, in most central office and outside plant applications, optical fiber is cabled, i.e. protected by one or both of a buffer material (typically silicon or epoxy resin) and a plastic jacket (usually PVC or polyethylene). Various cable types are common, but jacketed cables with a 3-mm diameter and tight-buffered fiber with a 900-micrometer nominal diameter are among the most common. It is not practical to apply micro-bends to such a jacketed or buffered fiber.
Macro-bends have a radius of curvature that is much larger than the waveguide diameter. When such a bend is applied to an optical fiber, it causes a significant amount of the light to leak out, even if the fiber is cabled. Moreover, in most cases, both the buffer coating and jacket materials have some degree of transparency at the near-infrared wavelengths used in telecommunication systems, permitting a non-negligible fraction of the leaked light to be detected externally.
For a given fiber, the degree of macro-bending required to detect a certain amount of leaked light is strongly dependent upon the wavelength. Less bending is required for longer wavelengths and more bending is required for shorter wavelengths. For a given wavelength, the required detection sensitivity is also dependent upon fiber type and the types and colors of the coatings and jackets protecting the fiber. The performance of a macro-bending device for signal detection is largely dependent upon the amount of light that is extracted, which is directly proportional to the induced attenuation of the guided light (“insertion loss”). Clearly, if the guided signal power is low, it is desirable to bend the waveguide to a relatively tight radius so as to extract as much signal as possible, but without exceeding the loss margin of the system, distorting the signal or even causing physical damage to the fiber. It is desirable, therefore, to control the bending radius carefully.
It is known for so-called “clip-on” devices to employ macro-bending means to extract a portion of the guided light from an optical waveguide and detection means to determine the amount of extracted light. However, almost all of the prior art is limited to macro-bending introduced by a fixed bending structure, i.e. where the bend radius is either not changed, or where a portion of the escaping light detected is not detected while the bend radius is changed (during the cable clamping process, for instance).
Among this fixed-bending-structure prior art, macro-bending devices are known which take account of the different fiber, jacket, and coating types. One such device is marketed by EXFO Electro-Optical Engineering Inc. under the product name LFD-200 live Fiber Detector. This clip-on device comprises a so-called “fixed” bending means with a set of interchangeable heads of different shapes and sizes over which the fiber is bent to a fixed bending radius. A disadvantage of this approach is that the insertion loss caused by this fixed macro-bending is strongly dependent upon wavelength, making it impractical to optimize both the insertion loss and sensitivity when the wavelength of the propagating light could be, say, 1310 or 1550 nm. Another disadvantage of this device is that the technician who is using the device has to change the device head manually for each fiber type. The use of the device is therefore limited to the set of cable types and wavelengths for which the set of interchangeable heads are designed.
In another known macro-bending device, disclosed in U.S. Pat. No. 4,671,653, the level of detected light is compared to a predetermined threshold determined by the fixed bending structure, and, if this threshold is reached, an indicator light turns on. A disadvantage of this device is that the level of detected light will be dependent upon factors such as the optical power in the fiber, the fiber type, and the wavelength, as well as the amount of absorption and scattering that occurs as the light passes through coatings and jackets. Hence, the choice of a threshold value is very approximate and depends, upon the particular application. Moreover, there is, in general, no information as to the macro-bending-induced insertion loss.
U.S. Pat. No. 5,781,285 discloses an optical fiber tap which, in order to avoid mechanical damage to the fiber, progressively decreases the bending radius of the fiber under test by means of a probe that pushes laterally against the fiber. The primary purpose of this device is to detect the presence or absence of light in the fiber. This device can be used with various fiber types, as several different concave areas having different radii of curvature may be offered on the bottom plate of the instrument to introduce respective macro-bends. A probe is used to push against the fiber, and a detector is located in the probe. It is noted that the use of different concave areas is functionally equivalent to the use of interchangeable heads in the aforementioned EXFO instrument. In the absence of knowledge of fiber type, wavelength, and optical power in the fiber, the macro-bending-induced insertion loss of this device could lead to excessive loss and hence possible system failure.
European patent No. 0639762 also discloses an optical fiber tap which applies a macro-bend, but makes use of a damping mechanism to limit the abruptness with which this macrobend is applied, thereby reducing the chance of fiber damage and, in the case of a live transmission system, thereby reducing the risk of an error burst during application. However, there is no information as to the macro-bending-induced insertion loss and this device could lead to excessive loss and hence possible system failure.
Other known macro-bending devices involve winding the optical fiber around a mandrel. European Patent No. 0361962 discloses such a fiber tap which winds the fiber around a conical mandrel. The bending radius is gradually reduced as the mandrel is rotated about its axis until some leaked light is detected by a detector, or until a predetermined maximum level of bending is reached. A disadvantage of this device is that, after the fiber has been bent enough for some light to be extracted, light is being leaked along a section of fiber that is longer than necessary for detection purposes. As a result, a large proportion of this light is not detected and is therefore wasted. As before, any measurements taken with this device must be calibrated for fiber type and wavelength, as well as jacket and buffer coating type, and, in the absence of any of this knowledge, it is not possible to derive the macrobend-induced insertion loss.
More recently, U.S. patent application Ser. No. 2005/0041902 (Frigo et al.) disclosed a method and apparatus for identifying an optical fiber by applying time-varying modulation to an optical signal propagating through the fiber. A transmitter, for example a vibrating piston contacting the fiber laterally, applies the time-varying modulation at a first location, and a downstream receiver extracts light by bending the fiber around a mandrel to extract a portion of the signal, and detects the time-varying modulation in the extracted portion.
As in the case of the aforementioned EXFO device, Frigo et al.'s primary embodiments use interchangeable mandrels or “anvils”, each characterized by a different radius of curvature. As explained above, the use of such mandrels is inconvenient in the field, particularly when the technician is not certain of the wavelength or power level in the fiber under test.
Frigo et al.'s primary aim is to detect the presence of the modulation signal on the light propagating in the fiber while minimizing the intrusiveness of the measurement. Consequently, they seek to extract a minimal amount of the light from the fiber, i.e., only enough to enable detection of the time-varying signal, so as to ensure that insertion loss limits are not exceeded. In the absence of knowledge of the optical power in the fiber, the fiber type and wavelength information, however, it is not possible to derive the actual value of the macrobend-induced insertion loss and, hence, be assured that insertion loss limits are not being exceeded.